U.S. patent application number 10/364664 was filed with the patent office on 2004-08-12 for transmission mask with differential attenuation to improve iso-dense proximity.
Invention is credited to Lin, Benjamin Szu-Min, Liu, Hang Yip, Schmidt, Sebastian.
Application Number | 20040157135 10/364664 |
Document ID | / |
Family ID | 32824475 |
Filed Date | 2004-08-12 |
United States Patent
Application |
20040157135 |
Kind Code |
A1 |
Liu, Hang Yip ; et
al. |
August 12, 2004 |
Transmission mask with differential attenuation to improve
iso-dense proximity
Abstract
An apparatus, system and method to compensate for the proximity
effects in the imaging of patterns in a photolithography process. A
light exposure of a photoresist layer is effectuated in
predetermined patterns through an exposure mask having
light-transmissive openings in correspondence to the predetermined
patterns. The exposure mask has areas densely populated with the
light-transmissive openings and areas sparsely populated with the
light-transmissive openings. Light is attenuated through the
densely populated light-transmissive openings by a different amount
than through the sparsely populated light-transmissive
openings.
Inventors: |
Liu, Hang Yip; (Montrose,
NY) ; Schmidt, Sebastian; (Dresden, DE) ; Lin,
Benjamin Szu-Min; (Hsinchu City, TW) |
Correspondence
Address: |
Jackson Walker LLP
Ste. 600
2435 N. Central Expwy.
Richardson
TX
75080
US
|
Family ID: |
32824475 |
Appl. No.: |
10/364664 |
Filed: |
February 11, 2003 |
Current U.S.
Class: |
430/5 ;
430/322 |
Current CPC
Class: |
G03F 7/70283 20130101;
G03F 1/36 20130101; G03F 1/50 20130101 |
Class at
Publication: |
430/005 ;
430/322 |
International
Class: |
G03C 005/00; G03F
009/00 |
Claims
What is claimed is:
1. A photomask apparatus, comprising: a transparent support glass;
a first light attenuator secured to said transparent support glass
and cooperable therewith for creating in a photoresist material a
first pattern of features having a first feature density, said
first light attenuator for imposing a first amount of attenuation
on light that passes through said transparent support glass; and a
second light attenuator secured to said transparent support glass
and cooperable therewith for creating in the photoresist material a
second pattern of features having a second feature density that
differs from said first feature density, said second light
attenuator for imposing on light that passes through said
transparent support glass a second amount of attenuation that
differs from said first amount of attenuation.
2. The apparatus of claim 1, wherein said first feature density is
greater than said second feature density, and said first amount of
attenuation is greater than said second amount of attenuation.
3. The apparatus of claim 1, wherein all of the features of said
first and second patterns are of substantially equal dimension.
4. The apparatus of claim 1, wherein said features correspond to
contact holes.
5. The apparatus of claim 1, wherein said first and second light
attenuators respectively overlie first and second portions of said
transparent support glass, and including a layer of reflective
material overlying a third portion of said transparent support
glass other than said first and second portions thereof.
6. The apparatus of claim 1, wherein said first and second light
attenuators respectively include first and second portions of a
layer of partial light shielding material overlying said
transparent support glass, and wherein said first and second
amounts of attenuation are determined respectively by the
thicknesses of the corresponding portions of said layer of partial
light shielding material.
7. The apparatus of claim 6, including a layer of reflective
material overlying a third portion of said layer of partial light
shielding material other than said first and second portions
thereof, wherein said first portion of said layer of partial light
shielding material is thicker than said second portion thereof, and
wherein said first feature density is greater than said second
feature density.
8. The apparatus of claim 6, including a layer of reflective
material overlying a third portion of said layer of partial light
shielding material other than said first and second portions
thereof, and wherein said first light attenuator includes a further
layer of partial light shielding material overlying said first
portion of said first-mentioned layer of partial light shielding
material.
9. The apparatus of claim 1, wherein said transparent support glass
is quartz.
10. The apparatus of claim 1 further including a phase shifter for
producing in light passing through a first portion of said
transparent support glass a phase shift relative to light passing
through a second portion of said transparent support glass, said
phase shifter cooperable with said first light attenuator for
creating in the photoresist material said first pattern of
features.
11. The apparatus of claim 1, wherein a first portion of said
transparent support glass has a thickness which is less than a
thickness of a second portion thereof, thereby to produce in light
passing through said first portion of said transparent support
glass a phase shift relative to light passing through said second
portion of said transparent support glass.
12. The apparatus of claim 1, wherein said first amount of
attenuation is based on said first feature density and said second
amount of attenuation is based on said second feature density.
13. A method of imaging patterns in a photolithography process,
comprising: effectuating a light exposure of a photoresist material
to create therein a first pattern of features having a first
feature density, including imposing a first amount of attenuation
on light produced by a light source; and effectuating a light
exposure of the photoresist material to create therein a second
pattern of features having a second feature density which differs
from said first a feature density, including imposing on light
produced by the light source a second amount of attenuation that
differs from said first amount of attenuation.
14. The method of claim 13, wherein said first feature density is
greater than said second feature density, and said first amount of
attenuation is greater than said second amount of attenuation.
15. The method of claim 13, wherein all of the features of said
first and second patterns are of substantially equal dimension.
16. The method of claim 13, including selecting said first amount
of attenuation based on said first feature density, and selecting
said second amount of attenuation based on said second feature
density.
17. The method of claim 16, wherein said step of selecting said
first amount of attenuation includes selecting said first amount of
attenuation based also on a dimension desired for the features of
said first pattern, and wherein said step of selecting said second
amount of attenuation includes selecting said second amount of
attenuation based also on a dimension desired for the features of
said second pattern.
18. The method of claim 17, wherein the dimension desired for the
features of said first pattern is equal to the dimension desired
for the features of said second pattern.
19. An imaging system, comprising: a radiation source; a radiation
sensitive material; and an exposure mask disposed between said
radiation source and said radiation sensitive material, said
exposure mask including radiation reflective areas and
radiation-transmissive openings, said exposure mask for exposing
said radiation sensitive material only to radiation which passes
through said radiation-transmissive openings, said exposure mask
having a first area wherein some of said radiation-transmissive
openings are distributed in a first density, and having a second
area wherein some of said radiation-transmissive openings are
distributed in a second density which differs from said first
density; and said exposure mask including a differential attenuator
cooperable with said radiation transmissive openings, said
differential attenuator for attenuating radiation passing through
the radiation transmissive openings of said first area by a first
amount of attenuation, and said differential attenuator for
attenuating radiation passing through the radiation-transmissive
openings in said second area by a second amount of attenuation that
differs from said first amount of attenuation.
20. The system of claim 19, wherein said first density is greater
than said second density, and said first amount of attenuation is
greater than said second amount of attenuation.
21. The system of claim 19, wherein said differential attenuator
includes a partial radiation shielding material having first and
second thicknesses thereof disposed in said radiation-transmissive
openings of said first and second areas, respectively, and wherein
said first thickness is greater than said second thickness.
22. The system of claim 21, wherein said first density is greater
than said second density.
23. The system of claim 19, wherein said differential attenuator
includes a first layer of partial radiation shielding material
disposed in said radiation-transmissive openings of said first and
second areas, and a second layer of partial radiation shielding
material disposed in said radiation-transmissive openings of said
first area.
24. The system of claim 23, wherein said first density is greater
than said second density.
25. The system of claim 19, wherein said first amount of
attenuation is based on said first density and said second amount
of attenuation is based on said second density.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field of the Invention
[0002] The present invention relates generally to photolithography
and, more specifically, to correction of photoresist bias between
isolated patterns and dense patterns.
[0003] 2. Description of Related Art
[0004] Photolithography is commonly used during formation of
integrated circuits on semiconductor wafers. More specifically, a
form of radiant energy (such as, for example, ultraviolet light) is
passed through a radiation-patterning tool (e.g. photomasks and
reticles) and onto a semiconductor wafer. Conventional photomasks
and reticles contain light-restrictive regions (i.e., totally
opaque or attenuated regions) and light-transmissive regions (i.e.,
totally transparent regions) formed in a desired pattern. A grating
pattern, for example, can be used to define parallel-spaced
conductive lines on a semiconductor wafer. The wafer is provided
with a layer of photosensitive resist material commonly referred to
as photoresist. Radiation is passed through the
radiation-patterning tool onto the layer of photoresist and the
features of the mask pattern are transferred to the photoresist.
The photoresist is then developed to remove either the exposed
portions of photoresist for a positive photoresist or the unexposed
portions of the photoresist for a negative photoresist. The
remaining patterned photoresist can then be used as a mask on the
wafer during a subsequent semiconductor fabrication step, such as,
for example, ion implantation or etching relative to materials on
the wafer proximate the photoresist.
[0005] A method of forming a radiation-patterning tool is to
provide a layer of light-restrictive material (such as, for
example, chrome) over a light-transmissive substrate (such as, for
example, a fused silicon such as quartz), and subsequently etch a
pattern into the light-restrictive material.
[0006] In a typical process of fabricating semiconductor circuitry,
a desired circuit pattern will be designed, and subsequently a
radiation-patterning tool will be formed to create the pattern. A
problem in forming the radiation-patterning tool is in correlating
particular pattern shapes desired in the integrated circuitry to
pattern shapes utilized in the tool. Specifically, a pattern shape
formed in a tool will typically not be identical to a pattern shape
generated with the tool. Light passing through a reticle tends to
be refracted and scattered by the chromium edges causing
interference patterns. This causes the projected image to exhibit
some rounding and/or other optical distortion. The problems become
especially pronounced in IC designs having feature sizes near the
wavelength of light used in the photolithographic process. If the
transfer of the mask pattern is not correct, it may introduce
variances that exceed the tolerance of the critical dimension (CD)
on the wafer.
[0007] This optical distortion (also known as the proximity effect)
can be compensated for, at least in part, by modifying any given
feature in the opposite direction to the expected bias. Thus, a
line that would otherwise come out too narrow can be drawn as wider
than its true width, etc. The overall nature and scope of these
corrections will vary with the particular photolithography process
that is being used.
[0008] One obvious way to avoid this phenomena is to relax the
design rules of the circuit such that the CDs are not at the
resolution limit of the exposure tool. However, increasing CDs,
yields larger die sizes. Larger die and circuit sizes have many
undesirable characteristics such as high defect densities and
slower circuit speeds. Also, not utilizing the exposure tool to its
fullest capabilities is not a cost effective practice.
[0009] Another solution to the proximity effect is the use of
optical proximity correction (OPC). OPC compensates for the
proximity effect by altering the mask image such that the resulting
pattern matches the desired pattern of the non-altered mask image.
Optical proximity correction involves adding dark regions to and/or
subtracting dark regions from a reticle design at locations chosen
to resolve the distorting effects of diffraction and scattering,
for example. Typically, OPC is performed on a digital
representation of a desired IC pattern. First, the digital pattern
is evaluated using software to identify regions where optical
distortion is apt to result. Optical proximity correction is then
applied to compensate for the distortion. The resulting pattern is
ultimately transferred to the reticle glass.
[0010] The OPC process is generally performed by scanning a digital
version of an IC layout design to identify feature dimensions,
interfeature spacing, feature orientation, etc. The scanning
process may proceed across the IC layout design in a rasterized
fashion to cover the entire pattern. In some IC layout designs, it
may also be necessary to conduct raster scans in the two or more
directions (e.g., horizontal, vertical, and one or more diagonal
directions). In some cases, the OPC computations may include
generating a detailed computer model of a reticle image known as a
Fast Aerial Image of Mask (FAIM). This image is then itself
evaluated to determine where to make reticle corrections.
[0011] However, a drawback is encountered in that the process of
performing OPC on modern IC layout designs having many features can
be computationally intensive. In fact, OPC can sometimes be too
great for even the most advanced computational resources.
Obviously, when FAIM models are used, the computational difficulty
increases significantly.
[0012] Consequently, a method and apparatus are desired which
eliminate the need for different photolithography masks to
compensate for bias between isolated and dense mask patterns. It
would also be desired that the method and apparatus eliminate the
need to use more complicated design and production of OPC masks to
compensate for the bias.
SUMMARY OF THE INVENTION
[0013] The present invention achieves technical advantages as an
apparatus, system and method of compensating for the proximity
effects in the imaging of patterns in a photolithography process in
which a light exposure of a photoresist layer is effectuated in
predetermined patterns through an exposure mask having
light-transmissive openings in correspondence to the predetermined
patterns. Light is attenuated differently through
light-transmissive openings associated with a dense pattern than
through light-transmissive openings associated with a sparse
pattern.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] For a more complete understanding of the present invention,
reference is made to the following detailed description taken in
conjunction with the accompanying drawings wherein:
[0015] FIGS. 1-1A illustrate, in cross section, the process for
etching contacts according to the prior art;
[0016] FIG. 2-2A illustrates a top view of a wafer following
photoresist contact hole printing having both a dense contact
pattern portion and a sparse contact pattern portion;
[0017] FIG. 3 illustrates shows tabulated results of simulated data
for a given photoresist processing time in accordance with an
embodiment of the present invention;
[0018] FIG. 4 shows a photomask structure in accordance with an
embodiment of the present invention;
[0019] FIG. 4A shows a photomask structure in accordance with
another embodiment of the present invention; and
[0020] FIG. 5 shown a photomask structure in accordance with
another embodiment of the present invention.
DETAILED DESCRIPTION
[0021] The numerous innovative teachings of the present application
will be described with particular reference to the presently
preferred exemplary embodiments. However, it should be understood
that this class of embodiments provides only a few examples of the
many advantageous uses and innovative teachings herein. In general,
statements made in the specification of the present application do
not necessarily delimit any of the various claimed inventions.
Moreover, some statements may apply to some inventive features, but
not to others. Throughout the drawings, it is noted that the same
reference numerals or letters will be used to designate like or
equivalent elements having the same function. Detailed descriptions
of known functions and constructions unnecessarily obscuring the
subject matter of the present invention have been omitted for
clarity.
[0022] FIG. 1 illustrates a conventional photolithography process
which includes a light source 11, a wafer 10 coated with
photoresist 18, and a conventional mask 19. The mask 19 allows full
light transmission 14 through its contact pattern, and is used to
define the etching windows on photoresist 18. In FIG. 1A, the
etching windows are defined after the development of photoresist
18, wherein there is no residual photoresist layer within the
etching windows 15. Typically anisotropical etching is used to etch
the contacts. The mask 19 typically includes a light-transmissive
quartz substrate 13 and a light-restrictive chromium layer 17
etched to define the contact pattern.
[0023] Semiconductor designs often require a wafer to have very
dense contact patterns (i.e. regions in which many contact areas
are nested together) and very sparse contact patterns that have few
contact areas. FIG. 2 shows a top view of a wafer 20 following
photoresist contact hole printing having both a dense contact
pattern portion 21 and a sparse contact pattern portion 23. FIG. 2
illustrates conventional results of photoresist contact hole
printing with a photomask having both a dense and a sparse contact
pattern. Note the contact etching window on the wafer 20 in the
sparse portion 23 is smaller than the contact etching windows in
the dense portion 21. If two pattern elements are closely adjacent,
each element emits scattering electrons to the neighboring element
from which it also receives scattering electrons.
[0024] When a conventional mask having both dense and sparse
contact patterns is illuminated, the intensity of the radiation
passing through the dense pattern is stronger than that of a sparse
pattern. This difference in intensity results in a bias between the
dense and sparse patterns in which the photoresist in a dense
contact receive more exposure and results in a larger contact
etching window than in the sparse areas. FIG. 2A illustrates ideal
results of photoresist contact hole printing in which contact
etching windows in the dense and sparse portions are substantially
the same size and shape.
[0025] An aspect of the present invention is to normalize the
intensity of the radiation, such that the resulting photoresist
pattern matches the desired pattern of the mask image without
altering the mask image. That is, the radiation passing through a
dense pattern is attenuated more than that of the sparse pattern,
such that the photoresist radiation exposure of both dense and
sparse patterns fall within the same intensity range, thereby
reducing or eliminating the bias caused by under or over
exposure.
[0026] Firstly, in a method to compensate for the proximity effect,
in accordance with exemplary embodiments of the present invention,
the corresponding percentage transmissivity needed for the dense
and sparse patterns which will result in similar size contact holes
(or other desired features in the post-development photoresist) is
determined from experimentation or simulation, for example. The
determined results can then be tabulated.
[0027] FIG. 3 shows tabulated results of simulated data for a given
photoresist processing time in accordance with an embodiment of the
present invention. The first column 31 lists the attenuated
transmission in a percentage value, column 33 correlates the
critical dimension of an isolated photoresist contact hole at the
determined percent attenuation, and column 35 correlates the
critical dimension of photoresist contact holes in a nested/dense
pattern. For example, if a photoresist contact hole of
approximately 240 nanometers is desired, an attenuation factor of
1% is selected from the table for the isolated contact (which has a
simulated result with a CD of approximately 237.8 nanometers) and
an attenuation factor of 6% is selected for the nested contacts
(which has a simulated result with a CD of approximately 243.2
nanometers). Using the same example, the next step is to tune a
photomask using the determined attenuations, such that a first
contact pattern has an attenuation of 1% and a second contact
pattern has an attenuation of 6%. Lastly, photoresist processing is
effectuated using the tuned photomask.
[0028] Referring now to FIG. 4 there is shown a photomask structure
in accordance with exemplary embodiments of the present invention.
The mask structure 40 includes a support glass 41) e.g., quartz); a
layer of chromium 43, which blocks the light completely; a layer of
partial light shielding material 42, which determines how much
light is to be transmitted by its thickness; a first contact
pattern 48; and a second contact pattern 47. The layer of chromium
43 and a portion of the partial light shielding material 42 over
the support glass 41 are etched at 46 to produce the first contact
pattern 48, which allows light transmission determined from the
thickness of the partial light shielding material 42 remaining in
the first contact pattern 48. The second contact pattern 47 has
only the chromium layer etched and allows light transmission
determined from the thickness of the partial light shielding
material 42. For the above-mentioned example, the first contact
pattern 48 is tuned for a sparse contact pattern and the second
contact pattern 47 is tuned for a dense contact pattern.
[0029] Referring now to FIG. 5 there is shown another photomask
structure in accordance with exemplary embodiments of the present
invention. The mask structure 50 includes a support glass 41; a
layer of chromium 43, which blocks the light completely; a layer of
a partial light shielding material 52, which determines how much
light is to be transmitted by its thickness; a first contact
pattern 58; and a second contact pattern 57. The layer of chromium
43 over the support glass 41 is etched to produce the first contact
pattern 58, which allows a determined light transmission. The
second contact pattern 57 has the chromium layer 43 etched and has
a second layer of a partial light shielding material 51. The
partial light shielding material layers (51 and 52) can be the same
material or different materials. FIGS. 4 and 5 illustrate
photomasks with only two different partial light contact patterns
however, photomasks with more than two different partial light
contact patterns are contemplated. In addition, FIGS. 4 and 5
illustrate only two approaches for tuning a photo mask although
other approaches are contemplated.
[0030] The present variable transmission mask and method can be
incorporated with conventional phase shift type masks. In
comparison with conventional reticles, phase shift masks (PSM)
typically incorporate shifter regions within the conventional
chrome metal-on-glass reticle construction. Typically,
phase-shifting is achieved by passing light through areas of a
transparent material of either differing thicknesses or through
materials with different refractive indexes or both thereby
changing the phase or the periodic pattern of the light wave.
Specifically, light rays passing through the transparent substrate
and the shifter regions have different optical path lengths, and
thus emerge from those surfaces with different phases in a manner
well known in the art. Application of the above-described variable
transmission pattern with a PSM, as shown, for example, in FIGS. 4A
and 4B, enables an attenuated PSM with multiple attenuation
regions. The thicker or thinner transparent regions can be
effectuated by etching 411 the support glass 41, by depositing a
layer of phase shift material 415 (such as silicon nitride), or by
a combination of both, to achieve a desired thickness that
corresponds to a desired phase shift. Such PSM techniques are known
in the art. These shifter regions are designed to produce a
thickness related to the wavelength of light passing through the
phase shift mask. In some embodiments of the present invention,
differing thicknesses of the attenuating material 42, as
above-described, are applied as shown in FIGS. 4A and 4B.
[0031] Although a preferred embodiment of the method and system of
the present invention has been illustrated in the accompanied
drawings and described in the foregoing Detailed Description, it is
understood that the invention is not limited to the embodiments
disclosed, but is capable of numerous rearrangements,
modifications, and substitutions without departing from the spirit
of the invention as set forth and defined by the following
claims.
* * * * *